1. Field of the Invention
This invention relates to a method of producing a lithographic photosensitive material and particularly of producing lithographic photosensitive materials which have improved characteristics with respect to pressure susceptibility.
2. Description of the Prior Art
When the photosensitive coating of a photographic material is subjected to pressure either by folding, curving or bending the material at a sharp angle or by scratching the material against a sharp substance, the area thereof subjected to these actions often exhibits an increased or decreased photographic sensitivity. These phenomena give rise to so called "pressure marks" or "scratch marks" fatally deteriorating the final photographic image. Hence, preventing the generation of pressure or scratch marks by reducing the susceptibility of a photographic material to pressure is desired.
Generally speaking, a photographic material designed to reproduce a line and half-tone dot image includes a certain cadmium compound to increase the photographic speed as well as the gamma. However, the cadmium salt dissolves into the processing solution during processing, thus causing a severe pollution problem with respect to processing wastes. Therefore photographic materials free of cadmium compounds are highly desirable. Unfortunately, those photographic materials which do not contain a cadmium salt not only tend to exhibit a slow speed and a low gamma value, but also are quite susceptible to pressure, with the above-mentioned "pressure or scratch marks" being formed very easily.
On the other hand, such a photographic material is usually developed with a so-called infectious developer whereby the presence of a polyoxyethylene compound can improve, as is well known, the dot quality and contrast characteristics. If, however, the polyoxyethylene compound is contained in the photographic material, an increased number of pressure or scratch marks due to a localized sensitivity reduction tends to appear, especially where the photographic material does not contain a cadmium salt. In order to suppress the susceptibility to pressure of a photographic material, the incorporation therein of any of a water soluble polymer, a latex of a water insoluble polymer and a humectic material such as an alcohol or glycerol as a gelatin plasticizer has been proposed. Although such a plasticizer can reduce the pressure marks caused by folding, curving or bending the material at a sharp angle, it is not effective to suppress scratch marks generated when the material is rubbed against a minute protrusion. In addition, the plasticizer tends to cause the gelatin layer after processing to be hazy and to make the surface of the coating tacky.